1. Field of the Invention
The present invention relates to a control device for a vehicle AC (alternating current) generator and, more particularly, relates to one which restricts an upper limit of a field current flowing through a field coil.
2. Description of the Related Art
In a known control device for a vehicle AC generator (Japanese Unexamined Patent Publication No. H3-45200), a resistor is connected in series to a field coil and a value of a field current flowing through the field coil is detected by a voltage across the resistor. Then, when the field current exceeds a predetermined value, the field current is cut off.
However, in a method in which a resistor is connected in series to a field coil for detecting a voltage across the resistor and a field current value is detected, if a resistance value of the resistor is large, loss at the resistor becomes large and a sufficient voltage cannot be applied to the field coil because of a voltage drop due to the resistor; and therefore, a resistor having a very small resistance value is used. But, it is difficult to secure accuracy of the very small resistance value and the resistance value is small; and thus a detected voltage is minute. In order to detect the current with high accuracy from the minute voltage, a detection circuit needs to have accuracy and a current sensor needs to have high accuracy, thereby becoming expensive.
Furthermore, if a field current is not restricted, an excessive current flows through a field coil when a power generation voltage of a self-excited AC generator becomes high; and consequently, it is likely to lead to burnout of the field coil or to burnout of the AC generator because a power generation output becomes excessive.